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Old 12-17-2022, 07:52 PM   #1
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Year: 1998
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Air system head aches

We bought a Thomas safetyliner mvp with a Cummins 8.3 a couple months ago, stripped it and built it out, loaded the family and pets and took off across the country.
The route we choose was beautiful but steep. All was working fine until we got to the bottom of our highest and longest pass yet. When we rolled into the small town at the bottom of the pass i found I had lost all throttle and the bus would not shift. After a short inspection I discovered I had some air leaks. No problem right? That was just the beginning. After fixing the air leaks I restarted and the bus shifted and I had throttle again. But it just kept building until it blew lines again. We changed the governor still blowing lines. I was told the problem was the air drier so I changed it still blowing lines. The mechanic I'm working with says I might have ice in the system so I need to get some deicer into the system and let it "work it's way through." After a few hours of letting the bus idle the line going to the air dryer was so hot that you can't touch it and the compressor still won't unload or the air drier purge. It just keeps working.
After doing some resurch I'm not sure if my bus had the right parts on it and if it doesn't why was it working if it does why is it not working now
My family and I would love a little advise from the hive.

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Old 12-18-2022, 03:53 PM   #2
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Does your air dryer have a built-in heater? If so, make sure it's getting power: it should keep the purge valve from freezing. How cold is it where you are?

Also, is air continuously coming out the purge valve? It sounds like the air compressor is making air all the time, but you said you replaced the D-2 air governor. Is it installed correctly? The D-2 is the only thing that tells the air compressor to unload and load.

John
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Old 12-19-2022, 08:03 AM   #3
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Are you doing the work or is a mechanic doing it?

If a mechanic is doing it, find a new one, because anybody familiar with air systems should be able to easily diagnose this without shot gun blasting parts at it. With replacing the governor and the air dryer it sounds like they're throwing stuff at it.

Also, you shouldn't be blowing lines, as there are pressure relief valves on the tanks, dryer, and compressor that should open before you get to that point. Also, what's the temp? With a dryer, you shouldn't have issues with freezing, but if it's junk and not drying then you might have moisture in the system.

If you're doing this, you need to familiarize yourself with how it's supposed to operate correctly, and then see what isn't working correctly on your bus.

These videos describe the system operation, and they're from bendix, who I believe manufactured your braking system. You can skip the video about trailer brakes, as it doesn't apply. There are also a few quirks in the system that are school bus specific, but you likely won't run into them.



Like john said, the governor is what controls pressure by turning on and off the compressor. If that's working correctly(check that it is, don't assume it is because it's new) the compressor unloader valve could be stuck causing the compressor to run on.
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Old 12-19-2022, 08:06 AM   #4
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If it's really cold out, I've had governors collect water/crud and freeze up.

Another member on here had a governor installed incorrectly once too, which caused them to have a similar situation to you in that the compressor never shut off. In their case, the governor was bolted to the compressor, but it had a pipe plug where it shouldn't have had one, and then didn't allow the governor to send air to the compressor to turn it off.
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Old 12-19-2022, 12:43 PM   #5
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On my "old Crown" I had problems with ice in my air system but I could not build air pressure when that happened. I agree that throwing expensive parts at the air system is not the answer. My experience is that the air dryer relief valve should release excessive pressure in the system.


I had a defective governor cause excessive pressure in the new Crown. I replaced the defective governor with a Bendix brand part. If you still have the original governor examine it to see which passages were connected and assure the replaced governor is the same. Bendix has a web page that show how their governor works and what the connections are.

One rule I follow is when I am replacing a part like a governor that I haven't replaced befoe I take a picture of it before I disconnect anything and look at the picture while doing the replacement and after replacement.

With the over-pressure you experienced I would consider replacement of all hoses that experienced the over-pressure.



Don't try to drive the bus until you get this fixed. If you do you will stop very suddenly when the next hose fails.
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Old 12-19-2022, 09:36 PM   #6
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Yes it has a heater and it's cold. We are in the rocky mountains lows are below zero. When the problem started it was near freazing.
Yes we replaced the governor. The mechanic did it for me I assume he did it right but I didn't see how the old one was plumbed. I am on the road and not at home or I would have done it myself.
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Old 12-19-2022, 09:58 PM   #7
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Thanks booyah45828 it is a joint venture. I started by hiring a mechanic to fix it then I worked on it for awhile by myself then back to the mechanic now back to me. I have know the basics of how an air system works but I've never worked on one before. I have been trying to learn what and how each component works. So thank you I have been looking for just this. As far as a new mechanic I'm in a small town,not lots of potions.
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Old 12-19-2022, 10:08 PM   #8
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Thanks everyone. I will look into all this. I was thinking that I would replace the lines that had leaked and try another governor after I resurch how it is supposed be plumbed. Your advise is very much appreciated.
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Old 12-20-2022, 07:50 AM   #9
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Well, if you're up to the task, and willing to learn, we can try and walk you through this.

The governor is suspect #1. The fact that it's new doesn't mean it isn't faulty. Governors can be mounted to the compressor, and they can also be remote mounted. I've had issues with the remote mount units, as they were installed as the lowest point in the system and accumulated moisture. Another member on here had his governor mounted to the compressor, and when they installed it they had a plug on the governor where it shouldn't be, which didn't allow the governor to work properly.

So see how your governor is set up, and then we'll go from there.
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Old 12-20-2022, 07:51 AM   #10
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Start a build thread too, the hive loves to see pictures, layouts, plans, trips, etc.
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Old 12-20-2022, 08:24 AM   #11
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Quote:
Originally Posted by Booyah45828 View Post
Start a build thread too, the hive loves to see pictures, layouts, plans, trips, etc.
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Old 12-20-2022, 09:25 AM   #12
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the other possibility is that the unloader valve is stuck.. I had an unloader valve stick Off so my system would fail to build air periodically.. its also possible for the unloader to stick so that regardless of the governor signal the unloader never cuts out.
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Old 12-21-2022, 12:02 AM   #13
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Thank you. I am up to it. I do have experience with mechanical stuff and a degree that says I'm a mechanic but I never did it for a living.
In the middle of all this I got a call telling me my mom had passed so I'm now ingaged in all that for a bit.. I will be back at my bus in a week or so. I will study up on all you sent me so when I get there I know more about what I'm looking at. Thanks again.
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Old 12-21-2022, 07:24 AM   #14
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Sorry for your loss.
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Old 12-21-2022, 07:29 AM   #15
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Oh and the line between the compressor and dryer will get hot. It's supposed to be either copper or teflon lined stainless braid hose because of the heat and vibration.
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Old 12-21-2022, 07:28 PM   #16
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In my little experience with dryers the ones I have worked on have a line going from the dryer to the unloader on the compressor. If it were to have any moisture in it the unloader wouldn't experience the pressure it needed to do it's job and run constantly, so a frozen line going to the unloader would cause this. But you said your dryer heater is not working so that is where I would look first. Or put some shop air to the unloader line and see if you can get it to cycle.
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Old 12-23-2022, 09:46 AM   #17
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your mechanic is a moron. find one that actually knows air systems
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